Optimization of dimensional stability and mechanical performance of thermally modified wood using cyclic-gradient thermal treatment

The limited enhancement in specific properties of wood achieved through conventional thermal modification (CoTM) does not adequately meet the diverse requirements of applications. Consequently, surface layer thermal modification (SLTM) with varying layer thicknesses is needed to balance wood's dimensional stability and mechanical strength. In this study, the cyclic-gradient thermal treatment was used to prepare SLTM wood with two target surface layer thicknesses: 6 mm (SLTM-6) and 12 mm (SLTM-12). Mass loss, color variation, dimensional stability, mechanical properties, chemical structure, and total time consumption were evaluated. Results demonstrated that the cyclic-gradient thermal treatment enabled controlled surface layer thickness. The dimensional stability and compressive strength of SLTM-6 and SLTM-12 samples were intermediate between at CoTM185 (treated at 185 degrees C) and CoTM215 (treated at 215 degrees C). The total time consumption reduced by more than 24.7 % compared to CoTM. Significant differences in mass loss and color were observed between surface and core layers. Higher degradation of hemicellulose and cellulose occurred in the surface layer, resulting in a darker color. Increasing surface layer thickness reduced the intensity difference between the surface and core layers, and improving the overall dimensional stability of the SLTM wood. Results of this study provided valuable guidance for further enhancement of the SLTM process and offers insights for developing a new generation of TM wood.